1. Field of the Invention
This invention relates to adjustable shaft mechanisms for adjusting the gap between a print head and a platen by adjusting the height of the shaft on which the print head carriage rides with respect to the platen.
2. Description of the Prior Art
It is typical in all dot matrix impact printers that it is necessary to adjust the gap between the print head and the platen. This has typically been done by moving the carriage shaft on which the print head carriage rides. In the prior art this has been accomplished in at least two manners. One, a lever method in which the shaft is pivoted by a lever mechanism which is then locked in place with a locking screw. This method is inexpensive but offers a very small mechanical advantage and fine adjustment since it is only in the area of 3:1. Hence, adjustment of 0.001 inches of shaft position requires a critical movement of the lever end of 0.003 inches. Also, locking the levers in place with a screw requires the use of a tool. A second method employs a non-concentric shaft. This method provides for fine adjustment of the head gap, but the non-concentric shaft is costly to produce. Also, the vertical adjustment of the shaft is combined with the horizontal displacement which causes the print line to be shifted up or down on the page.
U.S. Pat. No. 4,746,233 discloses a printing apparatus of the type in which the adjustment mechanism of the present invention would be useful. See column 6, line 35 and following for the motion of the print head carriage.
In accordance with an aspect of the present invention, it is an object to provide a mechanism for adjusting the gap between a print head and a platen in a dot matrix printer by camming the adjustable shaft up and down using a cam on each end of the printer frame.
Another object is to provide spring loading between the adjustable shaft member and the fixed guide member upon which the printer carriage rides employing a spring biasing the shaft and guide member together, such that when printing takes place the force vector on the shaft is up and the shaft is maintained in position.